As a typical example of a solar cell which converts solar light energy into power, there is known a solar cell which uses crystalline silicon (single-crystalline or polycrystalline) as a semiconductor substrate, a so-called crystalline silicon solar cell. As such a crystalline silicon solar cell, there is known, e.g., a single-side light receiving solar cell 10 shown in FIG. 2.
The solar cell 10 has an n-Si layer 16 formed on the light-receiving surface side of a p-type silicon substrate (Si wafer: p-Si layer made of p-type crystalline silicon) 11 by pn junction formation, and has, on the surface of the n-Si layer 16, a reflection prevention film 14 made of titanium oxide or silicon nitride which is formed by CVD or the like and front surface electrodes (light-receiving surface electrodes) 12 made of Ag which are formed typically by screen-printing and firing a silver paste. On the other hand, on the back surface side of the p-type silicon substrate (p-Si layer) 11, the solar cell 10 has back surface external connection electrodes 22 made of Ag which are formed by screen-printing and firing the silver paste similarly to the case of the front surface electrodes 12, and an aluminum electrode 20 which exhibits a so-called back surface field (BSF) effect.
Such an aluminum electrode 20 is formed on a substantially entire surface of the back surface by printing and firing an aluminum paste composed mainly of an aluminum powder. An Al—Si alloy layer (not shown) is formed during the firing, and aluminum is diffused into the p-type silicon substrate (p-Si layer) 11 and a p+ layer 24 is formed. Such a p+ layer 24, i.e., a BSF layer is formed, whereby photogenerated carriers are prevented form being re-coupled in the vicinity of the back surface electrode and an improvement in, e.g., open-circuit voltage or short-circuit current (Isc) is achieved.
Incidentally, in order to effectively realize the BSF effect, the aluminum electrode 20 needs to be formed so as to have a certain film thickness (e.g., 30 to 60 μm). On the other hand, for the reason of a reduction in the production cost of the solar cell or a reduction in the size of the solar cell module, the p-type silicon substrate (Si wafer) 11 thinner than conventional substrates, i.e., a reduction in the thickness of the solar cell element itself is requested.
However, the reduction in the thickness of the substrate (Si wafer) 11 encourages the occurrence of deformation such as warpage or bend in the silicon substrate (wafer) itself during firing for forming the aluminum electrode 20 by a difference in thermal expansion coefficient between the substrate 11 itself and the aluminum electrode 20. Accordingly, various countermeasures for preventing the occurrence of deformation such as the warpage or the like are conventionally taken.
For example, Patent Literature 1 proposes an aluminum-containing paste composition for forming an impurity layer or an electrode layer on a p-type silicon semiconductor substrate which contains silicon oxide or aluminum oxide.